Fix error handling with the combination of @asynchronous and @gen.coroutine.

Also make self.finish() optional for coroutines since we can finish
if the future resolves successfully.
Delete HTTPConnection._close_callback in between requests.

Previously the close callback would cause a reference to the previous
RequestHandler to be retained while waiting for the next request.
Bump version to 3.0b2
Merge branch 'master' of https://github.com/facebook/tornado